Almost sinister prettiness … Zooey Deschanel with Joseph Gordon-Levitt in (500) Days Of Summer

The glassy-eyed, almost sinister prettiness of Zooey Deschanel colours the spacey, detached mood of this romantic tragicomedy. Despite clever moments and Hornbyesque touches of melancholy, it's let down by sitcom cliches, and by being weirdly incurious about the inner life of its female lead. Joseph Gordon-Levitt plays Tom, a failed architectural student who now has a stupid job composing gooey messages for greeting cards; the day he sets eyes on Summer Finn, his boss's gorgeous new assistant – played by Deschanel – is the day he falls in love.

Their jokey, flirtatious relationship is based on a shared love of British bands such as the Smiths and on a giggly, conspiratorial feeling of superiority to the silliness and phoniness of everything that surrounds them. But the relationship, which lasts 500 days, is doomed, and Tom plays us scenes from random days, out of narrative order, to illustrate the painful business. The problem seems to be that he believes in love and she doesn't. Or is it that she just doesn't believe in being in love with him?

Agonisingly, Summer ends the relationship and appears relatively serene about it, asking if they can't be "friends" – always a shattering blow to the male romantic ego. (It is second only to what an attractive woman kindly says to Woody Allen in Play It Again, Sam: "You're very sweet but you're not sexy …") There is an excellent sequence in which, ages after their breakup, but immediately after a chance meeting during which the old magic seemed back on, Tom hopefully shows up for a party that Summer is throwing, but the action unfolds in split-screen, showing "Expectations" and "Reality" with gradual, sickening divergence. That is a very, very believable scene. But there are also some cutesy and unreal stock characters: Tom has two confidant-buddies from a 90s-era Friends-type comedy world and, annoyingly, an all-knowing kid sister whose wisdom and common sense are retch-inducing.

Deschanel's Summer herself is an enigma, and not always an intentional one. Her presence on screen is contoured in such a way as to be good-looking without being obviously sexy: often, she wears a very demure blue dress in translucent material over what appears to be an old-fashioned slip. Interestingly, she played a similar kind of guileless heartbreaker in David Gordon Green's All the Real Girls, but then her muddled motivations seemed more readable. Here, the film gives various clues to her increasing mismatch with Tom, but, frustratingly, Summer herself is a closed book. When she smiles, her cheeks plump up under her eyes, which maintain a calm, middle-distance gaze, and she glides around the place on castors – Stepford-bland and with the air of someone abusing prescription drugs or at any rate experiencing serious difficulties with her contact lenses.

It is a relief to have a film that tackles the painful side of romance, and this one has its good points. But I found myself squirming away from this film's embrace and murmuring: "It's not you, it's me …"